1. Field of the Invention
The present invention relates to an improved catalyst system for the ring-opening metathesis polymerization of cyclic olefins, such as dicyclopentadiene. More specifically, this invention relates to an improved co-catalyst for metathesis polymerization.
2. Description of Related Art
Cyclic olefins are subjected to ring-opening metathesis polymerization to produce thermoplastic or thermoset polymers having physical properties which make them suitable for structural and electronic applications, such as molded plastic items and electrical laminates. Such polymerizations are commonly carried out in reaction injection molding (RIM) processes, in which a metathesis catalyst and a monomer are charged to a heated mold, and polymerization of the monomer and forming of the polymer into the desired shape are carried out simultaneously in the mold.
In such RIM processes, it is important that the polymerization reaction occur rapidly and with as complete incorporation of the charged monomers as possible. For example, the presence of unreacted monomers in molded polydicyclopentadiene may result in a molded part with an unpleasant odor, and less than optimum physical properties. Finding a RIM process that reacts in as short a cycle time as possible and at mold temperatures at or near room temperature is economically desirable. It is also advantageous to be able to use a less than pure monomer stream and thus avoid extensive purification of the monomer prior to polymerization.
For most cyclic olefin metathesis catalyst systems, the presence of reactive materials such as water and oxygen should be avoided because they foul the polymerization reaction. Small amounts of reactive material may be acceptable, but water in excess of 20 parts per million (ppm) should generally be avoided. As a result, it is usually necessary to thoroughly dry the cycloolefin monomers prior to contact with the polymerization catalyst, and to conduct the polymerization in an inert environment.
Numerous patents and literature references describe techniques for polymerization of cycloolefins in the presence of a variety of olefin metathesis catalysts. Among the more effective ring-opening polymerization catalysts are homogenous catalyst systems based on tungsten or molybdenum halides, often employed with an organotin or organoaluminum co-catalyst. Examples of such catalyst systems are disclosed by Sjardijn et al., U.S. Pat. Nos. 4,810,762 and 5,093,441, wherein phenolic tungsten halides are used with organotin hydrides. Similar catalyst systems are disclosed by Sjardijn et al. in U.S. Pat. No. 4,729,976, which have been found to be highly active in a relatively impure dicyclopentadiene feed stream.
The use of aluminum or tin based co-catalysts, however, has certain drawbacks. Both co-catalysts require special handling because of toxicity concerns and sensitivity to air and moisture. Tin co-catalysts can also be expensive. Therefore, a co-catalyst for use with tungsten or molybdenum based catalysts other than tin or aluminum is desirable.